This invention relates to a screw joint tightening power tool of the type previously described in U.S. Pat. No. 4,951,756 and recited in the preamble of claim 1.
The power tool described in the above patent publication comprises a pneumatic vane motor supplied with pressure air via a supply valve. The latter is shifted from an ON-position to an OFF-position by a mechanism including an axial rod, a transverse latch member movably supported on the output shaft and an actuator cam supported on the driving clutch half. This actuator cam comprises a ring which is formed with a number of cam surfaces for interengagement with the latch member and which is freely rotatable over a limited angle relative to the driving clutch half so as to adjust automatically the interengagement point between the latch member and the cam surfaces to relative positions between the clutch halves where the maximum torque transferred by the clutch is just passed.
This control valve shifting mechanism has a serious drawback though, which has a negative influence on the accuracy of the maximum output torque level. This drawback is related to the ring shaped actuator cam which due to a relatively high inertia is too sluggish in action to provide a fast enough adjustment of the interengagement point between the cam surfaces and the latch member.